The invention relates to methods for treating hypercholesterolemia and atherosclerosis, and reducing serum cholesterol in a mammal. The methods of the invention comprise administering to a mammal a first amount of a bile acid sequestrant compound which is an unsubstituted polydiallylamine polymer and a second amount of a cholesterol-lowering agent. The first and second amounts together comprise a therapeutically effective amount.The invention further relates to pharmaceutical compositions useful for the treatment of hypercholesterolemia and atherosclerosis, and for reducing serum cholesterol. The pharmaceutical compositions comprise a combination of a first amount of an unsubstituted polydiallylamine polymer compound and a second amount of a cholesterol-lowering agent. The first and second amounts comprise a therapeutically effective amount. The pharmaceutical compositions of the present invention may optionally contain a pharmaceutically acceptable carrier.